1. Field of the Invention
The present invention relates to an exhaust gas processing apparatus for purifying an exhaust gas produced from an internal combustion engine.
2. Discussion of the Background
An electric discharge denitration apparatus (Jpn. Pat. Appln. KOKAI Publication No. 5-59934) shown in FIG. 1, an electric discharge denitration/dedusting apparatus (Jpn. Pat. Appln. KOKAI Publication No. 7-139338) shown in FIG. 2 and an electric discharge exhaust gas purifying apparatus (Jpn. Pat. Appln. KOKAI Publication No. 8-14028) are known as examples of the prior art for achieving a high purification performance even at the time of the start of operation of an internal combustion engine, wherein the electron temperature in exhaust gas is raised by means of a corona discharge and a catalyst is used together.
The electric discharge denitration apparatus 1 in FIG. 1 includes a discharge electrode 3 formed of a tungsten wire element 2, and a cylindrical receiving electrode 4. The discharge electrode 3 is disposed along a central axis of the cylindrical receiving electrode 4. The discharge electrode 3 is used as a cathode and the receiving electrode 4 as an anode. A high DC voltage is applied between the discharge electrode 3 and receiving electrode 4, an electric field is produced in a space 5 between both electrodes 3 and 4.
If exhaust gas to be processed is let into the receiving electrode 4, nitrogen oxides, hydrocarbons, etc. in the exhaust gas are electrified with negative ions 6 and adsorbed and decomposed by the receiving electrode 4. The exhaust gas is thus purified.
In the electric discharge denitration/dedusting apparatus in FIG. 2, exhaust gas 7 produced from a diesel engine for driving a generator is passed through a discharge tube 8 formed as a module, during which time the exhaust gas 7 is purified by atomized oil 9 and a corona discharge 13 applied by an AC power supply 12 between an external electrode 10 and a central electrode 11. On the basis of a detection output value of an exhaust gas sensor, the magnitude and frequency of an application voltage to the discharge tube 8 are varied for each discharge tube module.
The electric discharge exhaust gas purifying apparatus in FIG. 3 includes a receiving electrode 14 and a discharge electrode 15. The receiving electrode 14 is coated with a catalyst. Harmful components activated by electric discharge come in contact with the catalyst, and thus the rate of reaction for removing harmful matter is increased.
In the above prior-art apparatuses, one of the discharge electrodes is disposed in parallel to the direction of flow of exhaust gas, or the catalyst is divided and a voltage is applied therebetween.
In the above-described conventional method in which the corona discharge and catalyst are combined in use, it is necessary to create a non-equilibrium state in which the electron temperature is higher than the nucleus temperature. For this purpose, it is required to reduce the pulse width of electric current to about 100 ns or less and to cut off the current before electrons accelerated by an electric field deliver energy to atomic nuclei to enter an equilibrium state. In addition, it is necessary to set the frequency of pulses at 1 kHz or higher.
A thyratron, or one of vacuum tubes, is known as a device capable of performing such high-speed switching. The thyratron, though capable of performing high-speed switching, is not practical since its life as a high-speed switching device is about one month at most. An IGBT (Insulated Gate Bipolar Transistor) is widely used as a long-life, high-speed switching semiconductor device. The pulse width of this device, however, is merely about several-hundred ns and is unsatisfactory as a device for use in purifying exhaust gas.
Under the circumstances, there is a demand for an exhaust gas processing apparatus using a long-life, short-pulse high-repetition switching device, thereby achieving a high purification performance even when the temperature of exhaust gas is low at the time of the start of operation.
In general, in the above-described conventional methods in which the corona discharge and catalyst are combined in use, one of the discharge electrodes is disposed in parallel to the direction of flow of exhaust gas, or the catalyst is divided and a voltage is applied therebetween.
Where the electrode is disposed in parallel to the direction of flow of exhaust gas, an insulating member serving as an electrode support prevents a gas flow. As a result, the processing efficiency deteriorates, or serial connection for enhancing the purifying performance becomes difficult. On the other hand, where the catalyst is divided, the flow of discharged electrons does not traverse the exhaust gas and thus the electrons are not efficiently supplied into the exhaust gas.
It is thus desired that an exhaust gas processing apparatus with an enhanced exhaust gas purifying performance, wherein the electrode support does not prevent the flow of exhaust gas, be presented.